Process for the production of proteins

ABSTRACT

The invention pertains to the field of recombinant DNA technology and concerns a method for the production of a protein heterologous to yeast in a homogeneous form with the aid of Saccharomyces cerevisiae strain HT393 or a derivative thereof carrying a hybrid vector containing the genes for said protein.

The invention pertains to the field of recombinant DNA technology and concerns a method for the production of proteins with the aid of genetically engineered yeast cells carrying hybrid vectors comprising the genes for said proteins.

BACKGROUND OF THE INVENTION

Although in genetic engineering numerous protein expression systems for prokaryotic and eukaryotic hosts are already known, there is a continuing demand for novel systems which have advantages over the known systems.

Working on the expression of heterologous proteins in the baker yeast Saccharomyces cerevisiae, it has been commonly observed, that a high-level expression is dependent on many factors, e.g. plasmid stability, plasmid copy number, promoter strength, translation efficiency, low protein degradation.

In this context, one of the very important requisites is the yeast strain which is used for the production.

Recently, quite a number of heterologous proteins have been expressed in different yeast strains after transformation of yeast cells with suitable expression vectors comprising DNA sequences coding for said proteins, like e.g. α-interferon (Hitzeman et al. Nature (1981), 294, 717-722), tissue-type plasminogen activator (EP-A-143081) or certain desulfatohirudins (EP-A-225633). In many cases, however, the heterologous proteins are not synthesized in pure form, but as a mixture containing partially degraded such as C- or N-terminally shortened proteins. For instance, the expression of human atrial natriuretic peptide (hANP) in yeast resulted in the secretion of two forms of mature hANP differing in their C-terminus (Vlasuk et al. J. Biol. Chem. (1986), 261, 4798-4796). Similar results have been obtained after the expression of epidermal growth factor (EGF) in yeast (George-Nascimento et al. Biochemistry (1988), 27, 797-802) where the secreted expression products were heterologous in that either the last (Arg 53) or the last two amino acids (Leu 52 and Arg 53) were missing and no full-length EGF was produced.

The separation of mixtures containing full-length proteins such as α-interferon, tissue-type plasminogen activator, inhibitors of tissue-type plasminogen activator, or desulfatohirudins as well as partially degraded like C- or N-terminally shortened derivatives thereof into the individual components and the purification of these components to homogeneity, if these derivatives are biologically active at all, is laborious and time-consuming. Considering the incidental expenses there is a need for improved methods which render possible the economic production of homogenous proteins such as desulfatohirudin in yeast. It is an object of the present invention to provide methods for the production of proteins heterologous to yeast in a homogenous form.

Surprisingly it has been found, that the use of Saccharomyces cerevisiae strain HT393 for the expression of heterologous proteins leads to increased yield of biologically active and undegraded form of the expressed heterologous protein, compared to other Saccharomyces cerevisiae strains that are genetically closely related, e.g., to strain cl3-ABYS-86(DSM 9698) that is genetically closest related.

DESCRIPTION OF THE INVENTION

The present invention concerns a process for the production of a protein heterologous to yeast in a homogenous form characterized in that Saccharomyces cerevisiae strain HT393 (DSM 9697) or a derivative thereof is used for the expression of said heterologous protein.

In a preferred embodiment, the present invention relates to an improved process for the production of a protein heterologous to yeast in a homogenous form comprising culturing Saccharomyces cerevisiae strain HT393(DSM 9697) or a derivative thereof that has been transformed with a hybrid vector comprising a DNA sequence coding for said heterologous protein and isolating said heterologous protein.

A derivative of HT393 is a strain that is derived from HT393 and shows the same properties in respect to the production of heterologous proteins. The use of the inventive strains leads, e.g., to an increased yield of a biologically active and undegraded form of an expressed heterologous protein.

This heterologous protein can also be processed further, e.g. glycosylated. Useful proteins are, for example, enzymes that can be used, for the production of nutrients and for performing enzymatic reactions in chemistry, or proteins which are useful and valuable as nutrients or for the treatment of human or animal diseases or for the prevention thereof, for example hormones, polypeptides with immunomodulatory, anti-viral and anti-tumor properties, antibodies, viral antigens, vaccines, clotting factors, enzyme inhibitors, food-stuffs and the like.

Such heterologous structural genes are for example those coding for hormones such as secretin, thymosin, relaxin, calcitonin, luteinizing hormone, parathyroid hormone, adreno adenocorticotropin, melanoycte-stimulating hormone, β-lipotropin, urogastrone or insulin, growth factors, such as epidermal growth factor, insulin-like growth factor (IGF), e.g. IGF-I and IGF-II, mast cell growth factor, nerve growth factor, glia derived nerve cell growth factor, or transforming growth factor (TGF), such as TGFα or TGFβ, e.g. TGFβ1, β2 or β3, growth hormone, such as human or bovine growth hormones, interleukin, such as interleukin-1 or -2, human macrophage migration inhibitory factor (MIF), interferons, such as human α-interferon, for example interferon-αA, αB, αD or αF, β-interferon, γ-interferon or a hybrid interferon, for example an αA-αD- or an αB-αD-hybrid interferon, especially the hybrid interferon BDBB, inhibitors such as proteinase inhibitors such as α₁ -antitrypsin, SLPI, an inhibitor of the plasminogen activator (PAI-2) and the like, hepatitis virus antigens, such as hepatitis B virus surface or core antigen or hepatitis A virus antigen, or hepatitis nonA-nonB antigen, plasminogen activators, such as tissue plasminogen activator or urokinase, tumor necrosis factor, somatostatin, renin, β-endorphin, immunoglobulins, such as the light and/or heavy chains of immunoglobulin D, E or G, or human-mouse hybrid immunoglobulins, immunoglobulin binding factors, such as immunoglobulin E binding factor, e.g. sCD23 and the like, calcitonin, human calcitonin-related peptide, blood clotting factors, such as factor IX or VIIIc, erythropoietin, eglin, such as eglin C, desulfatohirudin, such as desulfatohirudin variant HV1, HV2 or PA, human superoxide dismutase, viral thymidin kinase, β-lactamase, glucose isomerase.

Preferred genes are those coding for a human α-interferon or hybrid interferon, particularly hybrid interferon BDBB and an inhibitor of the plasminogen activator (PAI-2).

In a preferred embodiment of the invention, the expressed heterologous protein is not secreted.

S. cerevisiae strain HT 393 (E95-1-2A) is obtained from strain cl3-ABYS-86 (DSM 9698) as described in Heinemeyer et al., EMBO J. (1991), 10, 555-562.

The wording derivatives of S. cerevisiae strain HT 393 embraces strains that are derived by genetic engineering from HT 393 and are, e.g., strains that are additionally cleared from two-micron (2μ) DNA (cir⁰), have a different mating type (MATα) and/or different selection marker. Preferred is HT 393 and derivatives thereof that are also deficient for protease A, protease B, carboxypeptidase Y, carboxypeptidase S and proteinase yscE.

Essentially preferred are HT 393 and derivatives thereof that show at least the following genetic characterization MATa, leu2-3, leu2-112, ura3Δ5, prb1-1, cps1-3, prc1-1, pra1-1 and pre1-1.

EXPRESSION CASSETTES

A suitable expression cassette comprises a promoter operably linked to a DNA sequence coding for the protein and to a DNA sequence containing transcription termination signals.

The expression cassette may additionally comprise a DNA sequence encoding a signal peptide linked in the proper reading frame to the DNA sequence coding for the inventive protein.

In a preferred embodiment, the promoter, the signal sequence, if present, and the terminator are recognized by the yeast expression system.

Promoter suitable for expression in a certain host are well known. Examples are the promoter of the TRP1 gene, the ADC1 gene (coding for the alcohol dehydrogenase I) or ADR2 gene (coding for the alcohol dehydrogenase II), acid phosphatase (PHO5) gene, CUP1 gene, isocytochrome c gene, or a promoter of the genes coding for glycolytic enzymes, such as TDH3, glyceraldehyde-3-phosphate dehydrogenase (GAPDH), a shortened version of GAPDH (GAPFL), 3-phosphoglycerate kinase (PGK), hexokinase, pyruvate decarboxylase, phosphofructokinase, glucose-6-phosphate isomerase, 3-phosphoglycerate mutase, pyruvate kinase, triosephosphate isomerase, phosphoglucose isomerase, invertase and glucokinase genes, a promoter of the yeast mating pheromone genes coding for the a- or α-factor, or the GAL/CYC1 hybrid promoter (intergenic region of the GAL1-GAL10 gene/Cytochrome1 gene; Guarente et al. Proc. Natl. Acad. Sci. (1982), 79, 7410-7414). Preferred vectors of the present invention contain, e.g., promoters with transcriptional control that can be turned on or off by variation of the growth conditions, e.g. the PHO5, the ADR2, or the GAL/CYC1 promoter. For example, the PHO5 promoter can be repressed or derepressed at will, solely by increasing or decreasing the concentration of inorganic phosphate in the medium.

The DNA sequence encoding a signal peptide ("signal sequence"), e.g. a yeast signal peptide, is derived, e.g., from a yeast gene, coding for a protein which is ordinarily secreted. Yeast signal sequences are, for example, the signal sequences of the yeast invertase (SUC2), α-factor, pheromone peptidase (KEX1), "killer toxin" and repressible acid phosphatase (PHO5) genes and the glucoamylase signal sequence from Aspergillus awamori. Additional sequences, such as pro- or spacer-sequences which may carry specific processing signals can also be included in the constructions to facilitate accurate processing of precursor molecules. For example, the processing signals contain a Lys-Arg residue, which is recognized by a yeast endopeptidase located in the Golgi membranes.

A DNA sequence containing transcription termination signals, e.g. yeast transcription termination signals, is preferably the 3' flanking sequence of a gene, e.g. a yeast gene, which contains proper signals for transcription termination and polyadenylation. The preferred flanking sequence is that of the yeast PHO5, the FLP and the α-factor gene.

The DNA coding for the protein according to the invention may be isolated from a gene bank of the natural host by methods known in the art like excision of the desired fragment using suitable restriction enzymes, PCR or may be synthesized chemically, using, e.g., the preferred codon usage of the host.

The promoter, the DNA sequence coding for the protein and the DNA sequence containing transcription termination signals are operably linked to each other, i.e. they are juxtaposed in such a manner that their normal functions are maintained. The array is such that the promoter effects proper expression of the protein or, if a signal sequence is present, the signal sequence-protein complex; the transcription termination signals effect proper termination of transcription and polyadenylation. In case a signal sequence is used, the signal sequence is linked in the proper reading frame to the protein gene in such a manner that the last codon of the signal sequence is directly linked to the first codon of the gene for the protein. The signal sequence, if present, has its own ATG for translation initiation. The junction of these sequences may, for example, be effected by means of synthetic oligodeoxynucleotide linkers carrying the recognition sequence of an endonuclease. Examples for related expression cassettes are described e.g. in EP-A-341215.

Preferred expression cassettes comprise the PHO5, the ADR2, or the GAL/CYC1 promoter, the DNA coding for a protein as defined above and the PHO5, α-factor or FLP terminator.

Especially preferred are expression cassettes as comprised, for example in pPAI-2A-10, pPAI-2A-20, pPAI-2B-10, pPAI-2B-20, or in SEQ ID NO:1 or a functional fragment or derivative thereof.

A functional fragment or derivative of said recombinant DNA molecule is, for example, a fragment coding for a shortened or elongated version of a above mentioned protein or an expression cassette containing a recombinant DNA molecule coding for said protein.

Recombinant Plasmids

The expression cassette as described above are normally inserted in a plasmid, suitable for expression of heterologous proteins in yeast. These plasmids are based, i.e., on the two-micron, pMB354 or the pEMBLyex plasmids (Cesareni and Murray, Genetic Engineering (1987), 4, 135-154). Suitable recombinant plasmids contain, for example, apart from the protein expression cassette, a DNA segment originating from two-micron DNA containing the origin of replication or total two-micron DNA. For example, plasmids according to the invention contain the complete two-micron DNA in an uninterrupted form, i.e. two-micron DNA is cleaved once with a restriction endonuclease, the linearized DNA is linked with the other components of the vector prior to recircularization. The restriction site is chosen such that normal function of the REP1, REP2 and FLP genes and of the ORI, STB, IR1 and IR2 sites of two-micron DNA as well as small "FLP recognition target" (FRT) sites, located near the center of each inverted repeat (IR) at which the FLP recombinase acts, are maintained. Optionally, the restriction site is chosen such that the D gene of two-micron DNA is kept intact too. Suitable restriction sites are, for example, the unique Hpal and SnaBl sites located outside of all of said genes and sites. However, it is likewise possible to insert the expression cassette and further components (cf. below) at different (such as two) restriction sites, especially those mentioned above, within two-micron DNA.

Such a plasmid derivative may comprise two invertedly repeated FRT sites or an additional, third FRT site. The former kind of plasmid is hereinafter called a "symmetric two-micron-like hybrid vector". The latter kind of plasmid is hereinafter called "symmetric two-micron-like disintegration vector" despite it is not a real symmetric plasmid but gives rise to a symmetric two-micron-like hybrid vector in the yeast cell transformed therewith.

A symmetric two-micron-like hybrid vector of the invention does preferentially not contain bacterial or viral DNA sequences, i.e. DNA derived from a bacterial genome, plasmid or virus. However, a two-micron-like disintegration vector of the invention may comprise DNA sequences of prokaryotic origin between the two directly repeated FRT sites which are excised from the vector in the transformed yeast cell in which the symmetric two-micron-like hybrid vector is generated from the disintegration vector. These DNA sequences are bacterial sequences as described below and can provide to the vector essential structural or functional features or can also only have the function of filling up the two regions between the two invertedly repeated FRT sites of an unsymmetric two-micron-like plasmid derivative or of an "unsymmetric" disintegration vector in order to construct a symmetric two-micron-like hybrid vector or a symmetric disintegration vector.

Preferably, the expression plasmids according to the invention include one or more, especially one or two, selective genetic markers, e.g. a marker for yeast and a marker and (except for symmetric two-micron like hybrid vectors) an origin of replication for a bacterial host, especially Escherichia coli.

As to the selective gene markers, any marker gene can be used which facilitates the selection for transformants due to the phenotypic expression of the marker gene. Suitable markers are, for example, those expressing antibiotic resistance or, in the case of auxotrophic yeast mutants, genes which complement host lesions. Corresponding genes confer, for example, resistance to the antibiotics G418, hygromycin or bleomycin or provide for prototrophy in an auxotrophic yeast mutant, for example the URA3, LEU2, LYS2, HIS3, or TRP1 gene.

As the amplification of the expression plasmids is conveniently done in a prokaryote, such as E. coli, a prokaryotic (e.g. E. coli), genetic marker and a prokaryotic (e.g. E. coli), replication origin are included advantageously. These can be obtained from corresponding prokaryotic plasmids, for example E. coli plasmids, such as pBR322 or a pUC plasmid, for example pUC18 or pUC19, which contain both prokaryotic, e.g. E. coli, replication origin and genetic marker conferring resistance to antibiotics, such as ampicillin.

Apart from the protein expression cassette, replication origin(s) and genetic marker(s) the expression plasmids according to the invention can contain optionally additional expression cassettes, such as 1 to 3 additional protein expression cassettes. The additional protein expression cassette(s) are identical to or different from each other and are identical to or different from the protein expression cassette already present on the vector.

Isolation of Proteins

The protein can be isolated by conventional means. For example, the first step consists usually in lysing or mechanically breaking the cell wall and removing the cell debris by centrifugation or, in the case of secretory proteins, in separating the cells from the culture fluid by means of centrifugation. The supernatant can be enriched for protein by treatment with polyethyleneimine so as to remove most of the non-proteinaceous material, and precipitation of the proteins by saturating the solution with ammonium sulfate or by extraction with a suitable solvent. Host proteins, if present, can also be precipitated, for example, by means of acidification with acetic acid (for example 0.1%, pH 4-5). Other purification steps include, for example, desalination, chromatographic processes, such as ion exchange chromatography, gel filtration chromatography, partition chromatography, HPLC, reversed phase HPLC and the like. The separation of the constituents of the mixture is also effected by dialysis, according to charge by means of gel electrophoresis or carrier-free electrophoresis, according to molecular size by means of a suitable Sephadex column, by affinity chromatography, for example with antibodies, especially monoclonal antibodies.

A further embodiment of the invention concerns the use of Saccharomyces cerevisiae strain HT393(DSM 9697) or a derivative thereof for the production of a protein heterologous to yeast in a homogenous form.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following experimental part various embodiments of the present invention are described with reference to the accompanying drawings in which:

FIG. 1 is a schematic illustration of plasmid pPAI-2A-10.

FIG. 2 is a schematic illustration of plasmid pPAI-2A-20.

FIG. 3 is a schematic illustration of plasmid pDP34R-PHO5-IFAM119.

DESCRIPTION OF THE YEAST STRAIN

    ______________________________________                                         Sc79     (Priece et al., Gene (1987), 55, 287-293)                                       XV2181 (Priece et al., Gene (1987), 55, 287-293)                       150-2B (Baldrai et al., EMBO (1987), 6, 229-293)                               CGY1465 (Rudolph et al., Cell (1989), 58, 1333-145)                            cl3-ABYS-86 (Heinemeyer et al., EMBO J. (1991), 10, 555-562) DSM                        9698 MATα, leu2-3, leu2-112, ura3.increment.5, prb1-1,                   cps1-3, prc1-1, pra1-1, his                                           HT 393 (Heinemeyer et al., EMBO J. (1991), 10, 555-562) DSM                     9697 MATa, leu2-3, leu2-112; ura3.increment.5, prb1-1,                         cps1-3, prc1-1, pra1-1, pre1-1                                              ______________________________________                                    

Examples

All DNA manipulations are--if not otherwise noted--carried out according to standard protocols, e.g. as described in Sambrook et al., Molecular Cloning: A laboratory manual, 2^(nd) Edn. Cold Spring Harbor Laboratory Press, Cold Spring Harbor N.Y., 1989.

1. Expression of Plasminogen Activator (PAI-2)

1.1 Construction of pMB354

This plasmid is based on the yeast/E. coli shuttle vector Yep24 and harbors the ADR2 promoter for the expression of heterologous proteins in S. cerevisiae.

The DNA sequence specific for the ADR2 gene is identical with the sequence published by Russell et al. (J. Biol. Chem. (1983), 258, 2674-2682) with the exception that a BamHI and a SalI restriction site are created upstream of the original ATG translation initiation codon of the alcohol dehydrogenase II protein. The mutated EcoRV/XbaI DNA fragment is cloned into pMac 5.8, yielding pMac 5.8-ADR (as described in Strassen et al., Nucl. Acids Res. (1985), 17, 4441-4454), which is digested with SmaI and XbaI. A 1100 bp fragment is isolated therefrom by cutting the plasmid with XbaI and EcoRI. This fragment is cloned into the vector Yep24, cut with NheI and PvulI yielding the new plasmid pMB354.

The relevant section of the cloned ADR2 promoter has the following structure: SEQ ID NO:5 ##STR1## 1.2. Isolation of cDNA Coding for PAI-2 SEQ ID NO:5

The cDNA for PAI-2 is derived from the U937 cell line codes for a protein with 415 amino acids and a predicted Mr. of 46.543 for the unglycosylated protein after removal of the initiator methionine. The protein has three potential N-glycosylation sites (Antalis et al., Proc. Natl. Acad. Sci. USA (1988), 86, 985-998). A suitable BgIII/EcoRI-DNA fragment comprising PAI-2 is isolated from the plasmid pBTA447 (Antalis et al., Proc. Natl. Acad. Sci. USA (1988), 85, 985-989). The protruding BgIII and EcoRI ends are made blunt-ended by the polymerase I Klenow fragment.

This allele type is termed A of PAI-2 (PAI-2A). There exists another allele with three amino acid exchanges at amino acid position

Asn₁₂₀ (AAT)→Asp₁₂₀ (GAT),

Asn₄₀₄ (AAC)→Lys₄₀₄ (AAG), and

Ser₄₁₃ (TCC)→Cys₄₁₃ (TGC), which is termed type B of PAI-2 (PAI-2B).

1.3. Cloning of PAI-1A

PAI-2 expression vectors pPAI-2A-10 and pPAI-2A-20 are based on the plasmid pEMBLyex4 (Cesareni and Murray, Genetic Engineering (1987), 9, 135-154). The construction of pEMBLyex4 and the necessary DNA elements are described in detail in this publication.

pEMBLyex4 is cleaved with SacI within the polylinker and the protruding SacI ends are made blunt-ended by polymerase I Klenow fragment prior to ligation with the isolated PAI-2 fragment (see above), resulting in the plasmid pPAI-2A-10 (FIG. 1).

In analogy, the PAI-2A cDNA is cloned into the BamHI/SalI site of plasmid pMB354 (example 1.1) to yield pPAI-2A-20 (FIG. 2).

1.4. Cloning of PAI-1B

The cDNA coding for PAI-2B is cloned in analogy to the cloning of PAI-2A cDNA into the polylinker of the plasmid pEMBLyex4 (see example 1.3) to yield pPAI-2B-10 and into pMB354 to yield pPAI-2B-20.

1.5. Transformation of Yeasts with pPAI-2A-10 and pPAI-2B-20

The plasmids pPAI-2A-10 and pPAI-2B-20 are transformed into six different strains:

    ______________________________________                                         Sc79     (Priece et al., Gene (1987), 55, 287-293)                               XV2181 (Priece et al., Gene (1987), 55, 287-293)                               150-2B (Baldrai et al., EMBO (1987), 6, 229-293)                               CGY1465 (Rudolph et al., Cell (1989), 58, 1333-145)                            cl3-ABYS-86 (Heinemeyer et al., EMBO J. (1991), 10, 555-562) DSM                        9698 MATα, leu2-3, leu2-112, ura3.increment.5, prb1-1,                   cps1-3, prc1-1, pra1-1, his                                           HT 393 (Heinemeyer et al., EMBO J. (1991), 10, 555-562) DSM                     9697 MATa, leu2-3, leu2-112, ura3.increment.5, prb1-1,                         cps1-3, prc1-1, pra1-1, pre1-1                                              ______________________________________                                    

1.6. Expression of PAI-2A and PAI-2B

PAI-2 can be detected in the soluble fraction of the cell extracts of all strains, but not in the sediment containing insoluble material nor in the culture broth. The expression of PAI-2 is analyzed by Western blotting, ELISA and biological activity testing.

Table 1 documents the expression yields of PAI-2A and PAI-2B in the different strains when cultivated in shaker flasks as described in the references.

                  TABLE 1                                                          ______________________________________                                                       Expression of                                                      Strain PAI-2A [mg/l] PAI-2B [mg/l]                                           ______________________________________                                         CGY1465       nd         0.1                                                     cl3-ABYS-86 31.5 14.5                                                          HT 393 50.8 23.7                                                               Sc79 0.4 nd                                                                    XV2181 14.1 nd                                                                 150-2B 32.0 nd                                                               ______________________________________                                          nd = not determined                                                      

The strain HT393 is the best producer compared to four other strains when the expression of PAI-2A is under the control of the GAL/CYC1 promoter, yielding 50 mg PAI-2A/I culture broth.

HT393 is also the best host for the expression of PAI-2B under the control of the ADR2 promoter.

In 1 I fermenters, the yields obtained by means of the plasmid pPAI-2A-10 are 310 mg PAI-2/1 in strain cl3-ABYS-86 and 680 mg PAI-2/l in strain HT393.

2. Expression of α-interferon B/D hybrids

2.1 Isolation of the cDNA clones for α-interferon subtypes B and D and construction of B/D hybrids

Human Namalva cells are induced for interferon synthesis by challenging the cells with Newcastle disease virus (NDV 10) under the standard conditions (EP-A-076489). Total mRNA is isolated and fractionated by sucrose density gradient centrifugation as described in EP-A-205404. Enriched fractions are used for cDNA synthesis. Cloning of the poly (dC) extended cDNA is done in the Pstl site of the E. coli vector pBR322. DNA sequence analysis allows assignment of the genes to the α-interferon subtypes as described by Henco et al. (J. Mol. Biol. (1985) 185, 227-260). Clones coding for polypeptides related to the α-interferon subtypes B and D are isolated.

The presence of common endonuclease restriction sites in subtypes B and D allows the subdividing of the protein coding region into four regions.

Restriction sites of the α-interferon coding region for the separation of these regions: ##STR2##

Combination of these subtype B and D regions gives access to 14 different B/D hybrids. The structural arrangement: B₁ D₂ B₃ B₄ is an α-interferon of the subtype B with the second region (amino acid residues 61 to 92) replaced by the corresponding region of subtype D. This sequence arrangement is obtained by ligating a restriction fragment containing the first region of the B gene with the rest of the D gene via the first common internal Sau3A restriction site to yield the hybrid B₁ D₂ D₃ D₄. The B/D hybrid B₁ D₂ B₃ B₄ is obtained by combining the N-terminal half of the B₁ D₂ D₃ D₄ gene with the C-terminal half of the B gene (B₁ B₂ B₃ B₄) (SEQ ID NO:2) via the common internal PvuII restriction site (EP-A-205404). The B/D hybrid B₁ D₂ B₃ B₄ is also referred to as IFAM119. The B/D hybrid structure is verified by restriction analysis using restriction endonucleases specific for the subregions of the original B and D genes. Final proof is obtained by DNA sequence analysis. DNA sequence data of the coding region of the hybrid gene B₁ D₂ B₃ B₄ are given in SEQ ID NO:1.

2.2. Construction of Expression Plasmid pDP34R/PHO5-IFAM119

Expression of α-interferon B/D is under the control of the repressible promoter of the yeast acid phosphatase gene (PHO5), which is derepressed by low concentrations of inorganic phosphate in the medium.

A BamHI restriction site at position -541 from the ATG of the PHO5 gene and an EcoRI linker inserted at position -8 of the gene allows convenient cloning of the 534 bp BamHI-EcoRI PHO5 promoter fragment. The PHO5 promoter is ligated to the coding sequence of α-interferon B/D (SEQ ID NO:4) via a synthetic oligodeoxynucleotide linker of the following sequence (see SEQ ID NO:1)

       EcoRI                DdeI                                                     5' AATTCGACAATGTGTGATCTGCC SEQ ID NO: 6                                         - 3'     GCTGTTACACACTAGACGGAGT                                        

The linker provides the EcoRI restriction site for ligation to the PHO5 promoter fragment, four non-transcribed nucleotides, ATG for translation initiation and the genuine (α-interferon B/D coding sequence up to the Ddel restriction site at position+14 from the ATG.

The PHO5-IFAM119 expression cassette of plasmid pDP34R/PHO5-IFAM119 consists of the 276 bp Sall-BamHI fragment of pBR322, the 534 bp BamHI/EcoRI PHO5 promoter fragment, the coding sequence of (α-interferon B/D (504 bp) and a 181 bp DNA fragment derived from the PHO5 transcription termination region (FIG. 1). The 3' end of the cassette is a HindIII site, which is filled in a reaction with Kienow DNA polymerase and is not cleavable upon cloning of the 1.6 kb SalI-HindIII blunt end expression cassette into the two-micron based yeast vector pDP34 (Hinnen et aL (1989) In Barr et al., (eds.) Yeast Genetic Engineering. Butterworths, Boston, pp. 189-213).

The nucleotide sequence of the complete expression cassette of plasmid pDP34R/PHO5-IFAM119 is determined by sequencing on double-stranded DNA using the chain-termination method according to Sanger. The DNA sequence is identical to the expected sequence (SEQ ID NO:1).

Expression plasmid pDP34R/PHO5-IFAM119 comprises the complete yeast two-micron circle, the yeast URA3 gene and the yeast dLEU2 gene. The dLEU2 gene does not have its own promoter (dLEU2≡leu2-d in Erhard et al., J. Bacteriol. (1983), 156, 625), therefore transcription of the dLEU2 gene depends on initiation in the 5' upstream two-micron sequences. The plasmid map is given in FIG. 3.

2.3. Expression of the B/D Hybrid

pDP34R/PHO5-IFAM119 is introduced into Saccharomyces cerevisiae HT 393 in the presence of polyethyleneglycol as described by Hinnen et al., (Proc. Natl. Acad. Sci. USA (1978), 1929). Host strain HT 393 is auxotrophic for leucine and uracil. The expression plasmid contains the dLEU2 and URA3 genes to complement the leucine and uracil auxotrophies of the host. Selection for plasmid-containing transformants is achieved by growth on a leucine supplemented, uracil deficient medium. The expression plasmid pDP34R/PHO5-IFAM119 is stably maintained in the host cell at a high copy number, even under non-selective growth conditions in complex medium.

α-interferon B/D is expressed under the control of the PHO5 promoter, which is induced at low concentrations of inorganic phosphate in the medium.

One single colony of the transformed HT 393 is picked into 10 ml of uracil deficient yeast minimal medium in a shakeflask and grown at 30° C. at 200 r.p.m. for 24 h. These cells are washed with minimal medium and used to inoculate uracil deficient medium for fermentation in a 500 ml shakeflask, in a 2000 ml flask and in a 30 I fermenter for 48 h each. Finally cells are grown for 47 h in a 300 I fermenter using low-P_(i) complex medium.

The produced interferon accumulates intracellularly in the cytosol of the cell.

2.4. Isolation of α-interferon B/D

5 g cells (from example 2.3) are collected by centrifugation, washed and adjusted with water and buffer (20% NaCl w/w and 8.3% KH₂ PO₄ w/w) to the following final composition:

Wet cell weight: 60% w/w

NaCl: 0.5 M

K-phosphate: 70 mM.

The pH is adjusted to pH 7.0 and the cells are broken by a single passage through a Dyno Mill® type KD-5 homogenizer filled with glass beads.

α-interferon B/D is isolated by 2 phase-extraction with

a) Extraction into the top phase using a two phase system comprising 7 parts of a solution of 9.5% K-phosphate and 12.5% NaCl (per weight); and 3 parts PEG 1500

b) Washing of the top phase using a solution of 20% K-phosphate and 5.8% NaCl

c) Back-extraction from the washed phase into the bottom phase using a solution of 18.8% MgSO₄ (per weight)

d) and subsequent reversed phase HPLC analysis.

Column: 150×4.6 mm C-18 (Vydac 218TP5415)

Buffer A: 0.1% trifluoro acetic acid in H₂ O

Buffer B: 80% acetonitrile containing 0.09% trifluoro acetic acid

Flow rate: 2.0 ml/min

Injection vol.: 50 μl.

Gradient: 61% solvent B to 100% Solvent B in 35 min.

Retention time for biological active α-interferon B/D: 22 min±1-2 min.

The amino acid composition of the isolated (α-interferon B/D was determined by gas phase hydrolysis and derivatisation of the amino acids with dimethylaminobenzenesulfonyl (DABS; Knecht & Chang, Analyt. Biochem. (1986), 58, 2375-2379) see table 2.

                  TABLE 2                                                          ______________________________________                                         Amino Acid       calculated                                                                              found                                                ______________________________________                                         Ala              9        9.1                                                    Arg 10 10.9                                                                    Asp 15 15.3                                                                    Cys 5 5.0                                                                      Glu 27 29.0                                                                    Gly 2 2.1                                                                      His 3 3.0                                                                      Ile 10 9.1                                                                     Leu 20 20.0                                                                    Lys 11 9.6                                                                     Met 5 4.3                                                                      Phe 10 9.6                                                                     Pro 4 4.8                                                                      Ser 15 13.7                                                                    Thr 6 6.8                                                                      Trp 2 nd                                                                       Tyr 5 4.9                                                                      Val 7 5.7                                                                    ______________________________________                                          nd = not determined                                                      

The N-terminal sequence of isolated α-interferon B/D is determined by automated Edman degradation in a gas phase sequencer and detection of the phenylthio-hydantoin amino acids by RP-HPLC. The experimentally determined sequence of the first 20 amino acids corresponds to the predicted sequence (SEQ ID NO:3).

The molar amounts of amino acids per mole of isolated α-interferon B/D determined experimentally by electrospray ionization mass spectroscopy (19566.6×0.27) are in agreement with the calculated values (19568.6).

The specific antiviral activity of isolated α-interferon is 1.5-2.3×10⁸ IU/mg protein (IU: international units) determined in the standard assay according to Rubinstein et al. (J. Virol. (1981), 37, 755) using MDBK cells and VSV (MDBK: Madin Darby bovine kidney; VSV: Vesicular stomatitis virus).

2.5. Comparison with another Yeast Strain

pDP34R/PHO5-IFAM119 is introduced into S. cerevisiae strain cl3-ABYS-86 (parental strain to HT393) and cultured as described above but with additional histidine in the culture broth. The α-interferon produced is isolated as described above (see Table 3).

                  TABLE 3                                                          ______________________________________                                                       Expression of                                                      Strain interferon [mg/l]                                                     ______________________________________                                         cl3-ABYS-86    50                                                                HT393 600                                                                    ______________________________________                                    

Strain HT393 transformed with pDP34R/PHO5-IFAM119 shows a ten fold increase in interferon expression compared to the transformed parental strain cl3-ABYS-86 transformed with pDP34R/PHO5-IFAM119.

Deposition of Microorganisms

The following microorganism strains were deposited at the Deutsche Sammiung von Mikroorganismen (DSM), Mascheroder Weg 1 B, D-38142 Braunschweig, Germany (accession numbers and deposition dates given):

    ______________________________________                                         Saccharomyces cerevisiae HT 393                                                                   DSM 9697   27.01.1995                                         Saccharomyces cerevisiae cl3-ABYS-86 DSM 9698 27.01.1995                     ______________________________________                                    

    __________________________________________________________________________     #             SEQUENCE LISTING                                                    - -  - - <160> NUMBER OF SEQ ID NOS: 6                                         - - <210> SEQ ID NO 1                                                         <211> LENGTH: 1566                                                             <212> TYPE: DNA                                                                <213> ORGANISM: Artificial Sequence                                            <220> FEATURE:                                                                 <221> NAME/KEY: misc.sub.-- feature                                            <222> LOCATION: (2)..(277)                                                     <223> OTHER INFORMATION: /function=pBR322 SalI-BamHI - #fragment              <220> FEATURE:                                                                 <221> NAME/KEY: promoter                                                       <222> LOCATION: (278)..(810)                                                   <223> OTHER INFORMATION: /phenotype=PHO5 promoter - #position -541 to -8       <220> FEATURE:                                                                 <221> NAME/KEY: misc.sub.-- feature                                            <222> LOCATION: (811)..(820)                                                   <223> OTHER INFORMATION: /function= EcoRI linker                               <220> FEATURE:                                                                 <221> NAME/KEY: CDS                                                            <222> LOCATION: (821)..(1324)                                                  <223> OTHER INFORMATION: /product=alpha interferon - #B/D including Met        <220> FEATURE:                                                                 <221> NAME/KEY: misc.sub.-- feature                                            <222> LOCATION: (1325)..(1380)                                                 <223> OTHER INFORMATION: /function= interferon 3'- # non-coding sequence       <220> FEATURE:                                                                 <221> NAME/KEY: terminator                                                     <222> LOCATION: (1381)..(1566)                                                 <223> OTHER INFORMATION: /standard name=linker and - # PHO5 terminator         <220> FEATURE:                                                                 <223> OTHER INFORMATION: Description of Artificial - #Sequence:recombinan            alpha interferon B/D with promoter - #and terminator                           sequences                                                                 - - <400> SEQUENCE: 1                                                          - - gtcgacgctc tcccttatgc gactcctgca ttaggaagca gcccagtagt ag -              #gttgaggc     60                                                                  - - cgttgagcac cgccgccgca aggaatggtg catgcaagga gatggcgccc aa -             #cagtcccc    120                                                                  - - cggccacggg gcctgccacc atacccacgc cgaaacaagc gctcatgagc cc -             #gaagtggc    180                                                                  - - gagcccgatc ttccccatcg gtgatgtcgg cgatataggc gccagcaacc gc -             #acctgtgg    240                                                                  - - cgccggtgat gccggccacg atgcgtccgg cgtagaggat ccgaaagttg ta -             #ttcaacaa    300                                                                  - - gaatgcgcaa atatgtcaac gtatttggaa gtcatcttat gtgcgctgct tt -             #aatgtttt    360                                                                  - - ctcatgtaag cggacgtcgt ctataaactt caaacgaagg taaaaggttc at -             #agcgcttt    420                                                                  - - ttctttgtct gcacaaagaa atatatatta aattagcacg ttttcgcata ga -             #acgcaact    480                                                                  - - gcacaatgcc aaaaaaagta aaagtgatta aaagagttaa ttgaataggc aa -             #tctctaaa    540                                                                  - - tgaatcgata caaccttggc actcacacgt gggactagca cagactaaat tt -             #atgattct    600                                                                  - - ggtccctgtt ttcgaagaga tcgcacatgc caaattatca aattggtcac ct -             #tacttggc    660                                                                  - - aaggcatata cccatttggg ataagggtaa acatctttga attgtcgaaa tg -             #aaacgtat    720                                                                  - - ataagcgctg atgttttgct aagtcgaggt tagtatggct tcatctctca tg -             #agaataag    780                                                                  - - aacaacaaca aatagagcaa gcaaattcga gaattcgaca atg tgt ga - #t ctg         cct      835                                                                                       - #                  - #        Met Cys Asp Leu Pro                           - #                  - #          1        - #       5        - - cag act cac agc ctg ggt aac agg agg gcc tt - #g ata ctc ctg gca caa           883                                                                        Gln Thr His Ser Leu Gly Asn Arg Arg Ala Le - #u Ile Leu Leu Ala Gln                             10 - #                 15 - #                 20               - - atg cga aga atc tct cct ttc tcc tgc ctg aa - #g gac aga cat gac ttt           931                                                                        Met Arg Arg Ile Ser Pro Phe Ser Cys Leu Ly - #s Asp Arg His Asp Phe                         25     - #             30     - #             35                   - - gaa ttc ccc cag gag gag ttt gat gat aaa ca - #g ttc cag aag gct caa           979                                                                        Glu Phe Pro Gln Glu Glu Phe Asp Asp Lys Gl - #n Phe Gln Lys Ala Gln                     40         - #         45         - #         50                       - - gcc atc tct gtc ctc cat gag atg atc cag ca - #g atc ttc aac ctc ttt          1027                                                                        Ala Ile Ser Val Leu His Glu Met Ile Gln Gl - #n Ile Phe Asn Leu Phe                 55             - #     60             - #     65                           - - acc aca aaa gat tca tct gct gct tgg gat ga - #g gac ctc cta gac aaa          1075                                                                        Thr Thr Lys Asp Ser Ser Ala Ala Trp Asp Gl - #u Asp Leu Leu Asp Lys             70                 - # 75                 - # 80                 - # 85        - - ttc tgc acc gaa ctc tac cag cag ctg aat ga - #c ctg gag tcc tgt gtg          1123                                                                        Phe Cys Thr Glu Leu Tyr Gln Gln Leu Asn As - #p Leu Glu Ser Cys Val                             90 - #                 95 - #                100               - - atg cag gaa gtg ggg gtg ata gag tct ccc ct - #g atg tac gag gac tcc          1171                                                                        Met Gln Glu Val Gly Val Ile Glu Ser Pro Le - #u Met Tyr Glu Asp Ser                        105      - #           110      - #           115                   - - atc ctg gct gtg agg aaa tac ttc caa aga at - #c act cta tat ctg aca          1219                                                                        Ile Leu Ala Val Arg Lys Tyr Phe Gln Arg Il - #e Thr Leu Tyr Leu Thr                    120          - #       125          - #       130                       - - gag aag aaa tac agc tct tgt gcc tgg gag gt - #t gtc aga gca gaa atc          1267                                                                        Glu Lys Lys Tyr Ser Ser Cys Ala Trp Glu Va - #l Val Arg Ala Glu Ile                135              - #   140              - #   145                           - - atg aga tcc ttc tct tta tca atc aac ttg ca - #a aaa aga ttg aag agt          1315                                                                        Met Arg Ser Phe Ser Leu Ser Ile Asn Leu Gl - #n Lys Arg Leu Lys Ser            150                 1 - #55                 1 - #60                 1 -       #65                                                                               - - aag gaa tga gacctggtac aacacggaaa tgattcttat agactaata - #c                  1364                                                                       Lys Glu                                                                         - - agcagctcac acttcgtcga gggtcagcag cgtcagtaac tctactgaat tg -              #accttcta   1424                                                                  - - ctgggactgg aacactactc attacaacgc cagtctattg agacaatagt tt -             #tgtataac   1484                                                                  - - taaataatat tggaaactaa atacgaatac ccaaattttt tatctaaatt tt -             #gccgaaag   1544                                                                  - - attaaaatct gcagccaagc tt           - #                  - #                    1566                                                                      - -  - - <210> SEQ ID NO 2                                                    <211> LENGTH: 167                                                              <212> TYPE: PRT                                                                <213> ORGANISM: Artificial Sequence                                            <220> FEATURE:                                                                 <223> OTHER INFORMATION: Description of Artificial - #Sequence:recombinan     t                                                                                     alpha-interferon B/D                                                      - - <400> SEQUENCE: 2                                                          - - Met Cys Asp Leu Pro Gln Thr His Ser Leu Gl - #y Asn Arg Arg Ala Leu         1               5 - #                 10 - #                 15               - - Ile Leu Leu Ala Gln Met Arg Arg Ile Ser Pr - #o Phe Ser Cys Leu Lys                    20     - #             25     - #             30                   - - Asp Arg His Asp Phe Glu Phe Pro Gln Glu Gl - #u Phe Asp Asp Lys Gln                35         - #         40         - #         45                       - - Phe Gln Lys Ala Gln Ala Ile Ser Val Leu Hi - #s Glu Met Ile Gln Gln            50             - #     55             - #     60                           - - Ile Phe Asn Leu Phe Thr Thr Lys Asp Ser Se - #r Ala Ala Trp Asp Glu        65                 - # 70                 - # 75                 - # 80        - - Asp Leu Leu Asp Lys Phe Cys Thr Glu Leu Ty - #r Gln Gln Leu Asn Asp                        85 - #                 90 - #                 95               - - Leu Glu Ser Cys Val Met Gln Glu Val Gly Va - #l Ile Glu Ser Pro Leu                   100      - #           105      - #           110                   - - Met Tyr Glu Asp Ser Ile Leu Ala Val Arg Ly - #s Tyr Phe Gln Arg Ile               115          - #       120          - #       125                       - - Thr Leu Tyr Leu Thr Glu Lys Lys Tyr Ser Se - #r Cys Ala Trp Glu Val           130              - #   135              - #   140                           - - Val Arg Ala Glu Ile Met Arg Ser Phe Ser Le - #u Ser Ile Asn Leu Gln       145                 1 - #50                 1 - #55                 1 -       #60                                                                               - - Lys Arg Leu Lys Ser Lys Glu                                                               165                                                             - -  - - <210> SEQ ID NO 3                                                    <211> LENGTH: 167                                                              <212> TYPE: PRT                                                                <213> ORGANISM: Artificial Sequence                                            <220> FEATURE:                                                                 <223> OTHER INFORMATION: Description of Artificial - #Sequence:recombina     nt                                                                                    alpha-interferon B/D                                                      - - <400> SEQUENCE: 3                                                          - - Met Cys Asp Leu Pro Gln Thr His Ser Leu Gl - #y Asn Arg Arg Ala Leu         1               5 - #                 10 - #                 15               - - Ile Leu Leu Ala Gln Met Arg Arg Ile Ser Pr - #o Phe Ser Cys Leu Lys                    20     - #             25     - #             30                   - - Asp Arg His Asp Phe Glu Phe Pro Gln Glu Gl - #u Phe Asp Asp Lys Gln                35         - #         40         - #         45                       - - Phe Gln Lys Ala Gln Ala Ile Ser Val Leu Hi - #s Glu Met Ile Gln Gln            50             - #     55             - #     60                           - - Ile Phe Asn Leu Phe Thr Thr Lys Asp Ser Se - #r Ala Ala Trp Asp Glu        65                 - # 70                 - # 75                 - # 80        - - Asp Leu Leu Asp Lys Phe Cys Thr Glu Leu Ty - #r Gln Gln Leu Asn Asp                        85 - #                 90 - #                 95               - - Leu Glu Ser Cys Val Met Gln Glu Val Gly Va - #l Ile Glu Ser Pro Leu                   100      - #           105      - #           110                   - - Met Tyr Glu Asp Ser Ile Leu Ala Val Arg Ly - #s Tyr Phe Gln Arg Ile               115          - #       120          - #       125                       - - Thr Leu Tyr Leu Thr Glu Lys Lys Tyr Ser Se - #r Cys Ala Trp Glu Val           130              - #   135              - #   140                           - - Val Arg Ala Glu Ile Met Arg Ser Phe Ser Le - #u Ser Ile Asn Leu Gln       145                 1 - #50                 1 - #55                 1 -       #60                                                                               - - Lys Arg Leu Lys Ser Lys Glu                                                               165                                                             - -  - - <210> SEQ ID NO 4                                                    <211> LENGTH: 166                                                              <212> TYPE: PRT                                                                <213> ORGANISM: Artificial Sequence                                            <220> FEATURE:                                                                 <221> NAME/KEY: PEPTIDE                                                        <222> LOCATION: (1)..(166)                                                     <223> OTHER INFORMATION: product=alpha interferon - #B/D                       <220> FEATURE:                                                                 <223> OTHER INFORMATION: Description of Artificial - #Sequence:alpha                interferon B/D without amino Met                                          - - <400> SEQUENCE: 4                                                          - - Cys Asp Leu Pro Gln Thr His Ser Leu Gly As - #n Arg Arg Ala Leu Ile         1               5 - #                 10 - #                 15               - - Leu Leu Ala Gln Met Arg Arg Ile Ser Pro Ph - #e Ser Cys Leu Lys Asp                    20     - #             25     - #             30                   - - Arg His Asp Phe Glu Phe Pro Gln Glu Glu Ph - #e Asp Asp Lys Gln Phe                35         - #         40         - #         45                       - - Gln Lys Ala Gln Ala Ile Ser Val Leu His Gl - #u Met Ile Gln Gln Ile            50             - #     55             - #     60                           - - Phe Asn Leu Phe Thr Thr Lys Asp Ser Ser Al - #a Ala Trp Asp Glu Asp        65                 - # 70                 - # 75                 - # 80        - - Leu Leu Asp Lys Phe Cys Thr Glu Leu Tyr Gl - #n Gln Leu Asn Asp Leu                        85 - #                 90 - #                 95               - - Glu Ser Cys Val Met Gln Glu Val Gly Val Il - #e Glu Ser Pro Leu Met                   100      - #           105      - #           110                   - - Tyr Glu Asp Ser Ile Leu Ala Val Arg Lys Ty - #r Phe Gln Arg Ile Thr               115          - #       120          - #       125                       - - Leu Tyr Leu Thr Glu Lys Lys Tyr Ser Ser Cy - #s Ala Trp Glu Val Val           130              - #   135              - #   140                           - - Arg Ala Glu Ile Met Arg Ser Phe Ser Leu Se - #r Ile Asn Leu Gln Lys       145                 1 - #50                 1 - #55                 1 -       #60                                                                               - - Arg Leu Lys Ser Lys Glu                                                                   165                                                             - -  - - <210> SEQ ID NO 5                                                    <211> LENGTH: 15                                                               <212> TYPE: DNA                                                                <213> ORGANISM: Artificial Sequence                                            <220> FEATURE:                                                                 <223> OTHER INFORMATION: Description of Artificial - #Sequence:ADR2                 promoter                                                                 <220> FEATURE:                                                                 <221> NAME/KEY: promoter                                                       <222> LOCATION: (1)..(12)                                                      <223> OTHER INFORMATION: portion of ADR2 promoter - #with BamHI and SalI             sites                                                                     - - <400> SEQUENCE: 5                                                          - - ggatccgtcg acatg              - #                  - #                       - #    15                                                                    - -  - - <210> SEQ ID NO 6                                                    <211> LENGTH: 23                                                               <212> TYPE: DNA                                                                <213> ORGANISM: Artificial Sequence                                            <220> FEATURE:                                                                 <223> OTHER INFORMATION: Description of Artificial - #Sequence:EcoRI/DdeI            linker                                                                   <220> FEATURE:                                                                 <221> NAME/KEY: misc.sub.-- feature                                            <222> LOCATION: (1)..(23)                                                      <223> OTHER INFORMATION: EcoRI/DdeI linker for lig - #ation to PHO5           promoter                                                                          - - <400> SEQUENCE: 6                                                          - - aattcgacaa tgtgtgatct gcc           - #                  - #                     23                                                                    __________________________________________________________________________ 

We claim:
 1. A process for the production of a protein heterologous to yeast comprising the steps of:introducing a gene encoding a protein heterologous to yeast into Saccharomyces cerevisiae strain HT393 or a derivative thereof; and, producing the protein heterologous to yeast in a homogenous form.
 2. The process according to claim 1 further comprising the steps of:culturing Saccharomyces cerevisiae strain HT393(DSM 9697), or a derivative thereof, that has been transformed with a hybrid vector comprising a DNA sequence coding for said heterologous protein, and isolating said heterologous protein.
 3. The process according to claim 1 wherein the heterologous protein is not secreted.
 4. The process according to claim 1 wherein the heterologous protein is selected from the group consisting of α-interferon, hybrid interferon and an inhibitor of the plasminogen activator.
 5. The process according to claim 1 wherein the heterologous protein is hybrid interferon BDBB or the inhibitor of the plasminogen activator PAI-2.
 6. The process according to claim 1 wherein the derivative of HT393 is cir° or MATα.
 7. The process according to claim 1 wherein the derivative of HT 393 is deficient for protease A, protease B, carboxypeptidase Y, carboxypeptidase S and proteinase yscE.
 8. The process according to claim 1 wherein the derivative of HT 393 is MAT α, leu2-3, leu2-112, ura3Δ5, prb1-1, cps1-3, prc1-1, pra1-1, pre1-1.
 9. The process according to claim 1 wherein the S. cerevisiae strain HT393 or a derivative thereof is transformed with a plasmid comprising an expression cassette, the expression cassette comprising a promoter operably linked to a DNA sequence coding for the protein and to a DNA sequence containing transcription termination signals.
 10. The process according to claim 9 wherein the promoter is selected from the group consisting of the PHO5, the ADR2, and the GAL/CYC 1 promoter.
 11. The process according to claim 9 wherein the transcription termination signal is selected from the group consisting of the yeast PHO5, the FLP and the α-factor terminator.
 12. The process according to claim 9 wherein the expression cassette comprises a recombinant DNA molecule as comprised in pPAI-2A-10, pPAI-2A-20, pPAI-2B-10, pPAI-2B-20 or in SEQ ID NO:1 or a functional fragment or derivative thereof.
 13. The process according to claim 9 wherein the plasmid is based on a plasmid selected from the group consisting of the two-micron, Yep24 and the pEMBLyex plasmids.
 14. A method for using Saccharomyces cerevisiae strain HT393(DSM 9697) or a derivative thereof comprising the steps of:introducing nucleic acid encoding a protein heterologous to yeast into Saccharomyces cerevisiae strain HT393(DSM 9697) or a derivative thereof; and, producing the protein heterologous to yeast in a homogenous form. 